1. Field of the Invention
The present invention relates generally to cable modem technology for receiving and transmitting data in a cable plant. More specifically, it relates to locating or detecting an appropriate data carrier in a downstream channel upon powering up a cable modem.
2. Discussion of Related Art
The cable TV industry has been upgrading its signal distribution and transmission infrastructure since the late 1980s. In many cable television markets, the infrastructure and topology of cable TV systems now include fiber optics as part of its signal transmission component. This has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cable commuting.
Originally, cable TV lines were exclusively coaxial cable. The system included a cable headend, i.e. a distribution hub, which received analog signals for broadcast from various sources such as satellites, broadcast transmissions, or local TV studios. Coaxial cable from the headend was connected to multiple distribution nodes, each of which could supply many houses or subscribers. From the distribution nodes, trunk lines (linear sections of coaxial cable) extended toward remote sites on the cable network. A typical trunk line is about 10 kilometers long. Branching off of these trunk lines were distribution or feeder cables (40% of the system's cable footage) to specific neighborhoods, and drop cables (45% of the system's cable footage) to homes receiving cable television. Amplifiers are provided to maintain signal strength at various locations along the trunk line. For example, broadband amplifiers are required about every 2000 feet depending on the bandwidth of the system. The maximum number of amplifiers that can be placed in a run or cascade is limited by the build-up of noise and distortion. This configuration, known as tree and branch, is still present in older segments of the cable TV market.
With cable television, a TV analog signal received at the headend of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 50 to 860 MHz. Broadcast signals were sent downstream; that is, from the headend of the cable system across the distribution nodes, over the trunk line, to feeder lines that led to the subscriber's home or premises. However, the cable system did not have installed the equipment necessary for sending signals from subscribers to the headend, known as return or upstream signal transmission. Not surprisingly, nor were there provisions for digital signal transmission either downstream or upstream.
In the 1980s, cable companies began installing optical fibers between the headend of the cable system and distribution nodes (discussed in greater detail in FIG. 1 below). The optical fibers reduced noise, improved speed and bandwidth, and reduced the need for amplification of signals along the cable lines. At many locations, cable companies installed optical fibers for both downstream and upstream signals. The resulting system is known as a hybrid fiber-coaxial (HFC) system. Upstream signal transmission was made possible through the use of duplex or two-way filters. These filters allow signals of certain frequencies to go in one direction and signals having different frequencies to go in the opposite direction. This new upstream data transmission capability allowed cable companies to use set-top cable boxes and allowed subscribers pay-per-view functionality, i.e. a service allowing subscribers to send a signal upstream through the cable system to the headend indicating that they want to see a certain program.
In addition, cable companies began installing fiber optic lines into the trunk lines of the cable system in the late 1980s. A typical fiber optic trunk line can be up to 80 kilometers long, whereas a typical coaxial trunk line is about 10 kilometers long. Prior to the 1990s, cable television systems were not intended to be general-purpose communication mechanisms. Their primary purpose was transmitting a variety of television signals to subscribers. Thus, there had to be one-way transmission paths from a central location, known as the headend, to each subscriber's home, delivering essentially the same signals to each subscriber. HFC systems run fiber deep into the cable TV network offering subscribers more neighborhood specific programming by segmenting an existing system into individual serving areas having between 100 to 2,000 subscribers. Although networks using exclusively fiber optics would be optimal, present cable networks equipped with HFC configurations are capable of delivering a variety of high bandwidth, interactive services to homes for significantly lower costs than networks using exclusively fiber optic cables.
FIG. 1 is a block diagram of a two-way HFC cable system utilizing a cable modem for data transmission. It shows a headend 102 (essentially a distribution hub) which can typically service about 40,000 subscribers. Headend 102 contains a cable modem termination system (CMTS) 104 needed when transmitting and receiving data using cable modems. Headend 102 is connected through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108.
Each headend can typically support up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the headend and each distribution node. In addition, because cable modems were not used, the headend of pre-HFC cable systems did not contain a CMTS. Each of the fiber nodes 108 is connected by a coaxial cable 110 to duplex filters 112 which permit certain frequencies to go in one direction and other frequencies to pass in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node 108 can normally service about 500 subscribers, depending on the bandwidth. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along trunk line 116, and subscriber taps 118, i.e. branch lines, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122 or other appropriate device.
As briefly mentioned above, recently it has been contemplated that HFC cable systems can be used for two-way transmission of digital data. The data can be Internet data, digital audio data, or digital video data, in MPEG format, for example, from one or more external sources 100. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. Most notably, they provide up to a thousand times faster transmission of digital data than is currently possible over telephone lines. However, in order for a two-way cable system to provide digital communications, subscribers must be equipped with cable modems, such as cable modem 120. With respect to Internet data, the public telephone network has been used, for the most part, to access the Internet from remote locations. Through telephone lines, data is typically transmitted at speeds ranging from 2,400 to 56,600 bits per second (bps) using commercial (and widely used) data modems for personal computers. Using a two-way HFC system as shown in FIG. 1 with cable modems, data can be transmitted at speeds of 10 million bps, or more. Table 1 is a comparison of transmission times for transmitting a 500 kilobyte image over the Internet.
TABLE 1Time to Transmit a Single 500 Kbyte ImageTelephone Modem (28.8 kbps)  6-8 minutesISDN Line (64 kbps)1-1.5 minutesCable Modem (10 Mbps)1 second or less
Furthermore, subscribers can be fully connected twenty-four hours a day to services without interfering with cable television service or phone service. The cable modem, an improvement of a conventional data modem, provides this high speed connectivity and, therefore, is instrumental in transforming the cable TV system into a full service provider of video, voice and data telecommunications services.
As mentioned above, the cable TV industry has been upgrading its coaxial cable systems to HFC systems utilizing fiber optics to connect headends to fiber nodes and, in some instances, using them in the trunk lines of the coaxial distribution system. In way of background, optical fiber is constructed from thin strands of glass that carry signals longer distances and have a wider bandwidth than either coaxial cable or the twisted pair copper wire used by telephone companies. Fiber optic lines allow signals to be carried much greater distances without the use of amplifiers (item 114 of FIG. 1). Amplifiers degrade the signal quality and can be expensive to maintain. Thus, coaxial distribution systems that use fiber optics have much less need for amplifiers. In addition, amplifiers are typically not needed for fiber optic lines (item 106 of FIG. 1) connecting the headend to the fiber nodes.
In cable systems, digital data is carried over radio frequency (RF) carrier signals. Cable modems are devices that convert a modulated RF signal to digital data (demodulation) and converts the digital data back to a modulated RF signal (modulation). The conversion is done at two points: the subscriber's home by a cable modem and at a CMTS located at the headend. The CMTS converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the operations are reversed. The digital data is input from a computer to the cable modem which converts it to a modulated RF signal. Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
As mentioned above, cable modem technology is in a unique position to meet the demands of users seeking fast access to information services, the Internet and business applications, and can be used by those interested in cable commuting. Not surprisingly, with the growing interest in receiving data over cable network systems, there has been a sharper focus on performance, reliability, and improved maintenance of such systems. Consequently, cable companies are in the midst of a transition from their traditional core business of entertainment television programming to being full service providers of video, voice and data telecommunication services. Among the elements that have made this transition possible are technologies such as the cable modem.
When a cable modem is turned on, it must first identify a viable data carrier on the downstream channel. The downstream channel transmits data from the CMTS, specifically the downstream transmitter, to the cable modems. A data carrier is a frequency channel typically 6 MHz wide in the United States and 8 MHz wide in Europe. Until the cable modem identifies the exact frequency channel corresponding to the data carrier, it cannot receive or transmit data. Present techniques for detecting a viable data carrier are inefficient, impractical, and take too much time upon powering up a cable modem.
There are two commonly used techniques for a cable modem to locate a viable data carrier. The most direct and efficient technique is preconfiguring the cable modem to immediately identify the correct data carrier in a particular geographical area upon power up. The cable modem is connected to the cable plant within the particular geographical area in which it will be sold and used, and is given time to detect the correct data carrier (methods for locating the data carrier are described below). Once the correct data carrier has been identified, the cable modem is configured to “tune in” on that frequency channel when turned on by a subscriber. However, preconfiguring cable modems in this manner will not be practical or cost-effective once cable modems become more widespread. Eventually, cable modem manufactures will assemble cable modems at one or more manufacturing facilities and ship them out to locations across the country. Preconfiguring individual cable modems for particular geographical areas will soon become cumbersome, impractical, and error-prone.
Another technique for locating the correct data carrier frequency channel is having the cable modem check each potential frequency as provided for in a frequency plan. A frequency plan is a list of frequencies used in a particular geographical area in a cable plant. Presently, there are about six frequency plans in use across the country. However, the number of plans is increasing and will likely keep growing as the number of subscribers increases. For example, a frequency plan can specify that each frequency channel starting at every 6th MHz, beginning with 88 MHz is a potential data carrier. The cable modem checks all frequency channels according to the list to locate the correct data carrier (there will only be one from the list that works for a particular geographical area, the list itself is used by one cable plant that covers several geographical areas). However, searching the list for the correct data carrier can take up to 20 minutes. This is because it can take up to 500 milliseconds to check each potential channel.
Related to this technique is configuring the cable modem to search all potential frequency channels from all frequency plans in order to find the correct channel. While this makes the cable modem more “generic” in that it can be used in any market or geographical area, the search time can exceed 40 minutes. Moreover, as mentioned above, the number of frequency lists will very likely grow in the future making this technique impractical. It is also possible that a cable company sets the downstream transmitter in a CMTS to the wrong frequency or to a frequency that is not included in a particular frequency plan. Even if the frequency is off by as little as 250 kHz, the cable modem will not register the closest frequency channel as a viable one.
Finally, searching one or more frequency lists can be avoided completely if the cable modem simply searches the entire downstream channel (i.e., from 88 MHz to 860 MHz in the United States, and 50 MHz to 1 GHz in Europe) in 250 kHz increments. By searching all frequency channels starting every 250 kHz, a cable modem can eventually locate the correct data carrier. Many cable modems in use today use this search method but the search time, sometimes well over 40 minutes, is unacceptable.
Therefore, it would be desirable to have a cable modem locate a correct and viable data carrier in its cable plant in as short a time as possible. Subscribers using cable modems should be able to power up a cable modem and use it for digital data transmission preferably within a few minutes. It would also be desirable to shorten the search time needed to locate a viable data carrier while eliminating the need to search one or more frequency plans, thereby making the cable modem ready to be installed in any geographical location. In addition, the search time should be shortened without having to preconfigure the cable modem for a particular geographical area before being used by subscribers.